Solvent extraction processes for the recovery of metals, and particularly uranium, from aqueous acid solutions, such as crude, industrial, 30% phosphoric acid solutions produced from phosphate rock, are well known. Typically, "brown", wet process phosphoric acid solution will contain about 600 grams/liter of phosphate, about 15 grams/liter of sulfate, about 9 grams/liter of iron, about 0.1 to 0.2 gram/liter of uranium and about 1 to 10 grams/liter of decayed organic material, organic humic acids, and other sludge-forming contaminant solids. These sludge-forming contaminant solids are soluble in the acid or are extremely fine, and will pass through the finest of filters. Accordingly, it is not technically feasible to fine-filter the feed acid solution to remove all of the contaminant solids.
The sludge-forming contaminants in the acid solution cause stable sludge emulsions during metal extraction. These emulsions will form and collect at the organic-aqueous interface, generally in the settler, in any solvent extraction process that uses organic metal extractant. The volume of sludge emulsion formed is over about 1.5 gallons/1,000 gallons of acid solution feed. If the sludge is not removed, flooding of the settlers results, and the equipment must be shut down.
The sludge emulsion is comprised of only about 2% to about 7% by volume of organic contaminant solids, with the rest of the emulsion being about equal volumes of acid solution and organic extractant. Simply discarding the sludge would result in the loss of expensive organic extractant, such as di(2-ethylhexyl) phosphoric acid (D2EHPA) - trioctylphosphine oxide (TOPO), plus the acid, including the uranium and other metals contained in both the acid and extractant solution. The cost for extractant and acid lost would be about $10/1,000 gallons of acid feed solution.
Reese et al., in U.S. Pat. No. 4,087,512, filters the sludge at room temperature. There, the sludge is fed directly into the filter from the settler, without the addition of any materials. This process recovers only about 60 to 70 vol. % of the extractant and acid from the emulsion, since the extractant, acid, and organic contaminant components are still in a stable emulsion form and intimately bound together. More importantly, the rate of filtration is extremely low, slowing the whole operation unless the emulsion is withdrawn from the process, stored, and then filtered at a later date.
Emulsion breaking is known in the art. Averill, in U.S. Pat. No. 1,617,739, relating to water-in-oil emulsions, teaches addition of 1 to 4 parts of water per 1 part water-in-oil emulsion. Then, a surface-active agent, such as gelatine, modified glue or gum acacia, is added and the temperature of the mixture is raised to at least 70.degree. C. This causes the emulsion to break and separate into its component parts.
Akell, in U.S. Pat. No. 3,644,214, teaches breaking liquid-liquid interface emulsions, such as oil-water, aniline-water, or methanol-water-xylol, without addition of surface active agents. Akell teaches adding an excess of one liquid component of the emulsion, in order to shift the composition into a single phase, according to the solubility diagram for the component system. If any solids, such as dirt and rust, are present after the single phase is formed, they would be filtered from the liquid using conventional techniques. Heating is considered impractical where there are large process throughputs, and is considered detrimental if one or more of the components are subject to reaction. No surface-active agents are added because they are expensive and are considered to constitute foreign materials in the system.
What is needed, to break the sludges formed by phosphoric acid solvent extraction, is a process for breaking up a three-component emulsion, containing a substantial amount of organic solids in an acidic-organic extractant medium. The process should allow quick filtration, and recover at least 90 vol. % of the acid and organic extractant as separate liquids from the sludge emulsion. No contaminating surface-active agents should be required.